The present invention relates to a process for preparing aromatic amines from aromatic hydrocarbons in which the water of reaction generated during hydrogenation of the nitroaromatic compounds is used to wash the nitrated aromatic compounds obtained by nitrating the corresponding aromatic hydrocarbons.
Aromatic monoamines and diamines, which are used in large amounts as chemical intermediates, for example to produce dyes or plastics, are prepared on an industrial scale by nitrating aromatic hydrocarbons to form nitroaromatic compounds and then hydrogenating the nitroaromatic compounds. In this case, one mole of water per mole of nitro group produced is generated during nitration with nitric acid and two moles of water per mole of amine group are generated during hydrogenation. This water ultimately leaves the process as waste water. Other waste water streams are also produced, particularly during working up of the nitroaromatic compounds which is conventionally performed by separating the acid phase and then washing the nitroaromatic compounds with water and alkaline solutions, generally stepwise and sometimes alternately. This waste water has to be purified in a suitable manner in order to be able to discharge it into the receiving waters in accordance with the pertinent regulations. Thus, for example, U.S. Pat. No. 6,506,948 describes a purification process for waste water which is produced during the preparation of dinitrotoluene.
EP-A-236 839 describes a distillation process for working up aqueous amine solutions such as those generated during the hydrogenation of nitroaromatic compounds. According to EP-A-236 839, the water being produced during distillation can be re-used in chemical processes. However, the concentration of organic impurities (in particular amines) in this water being produced during distillation is in the range from 10–500 ppm. Therefore, the use of this water in chemical processes frequently presents problems because these impurities can lead to difficulties in the processes in which they are present in the re-used water. For example, such re-used water may result in unwanted secondary reactions.
Diaminotoluene, in particular, reacts with dinitrotoluene to give unwanted secondary products, which is expressed, inter alia, in decreasing thermal stability of the dinitrotoluene.